The present invention relates to a resin composition having improved coating properties and more particularly to a resin composition having improved coating properties which is composed of a specific ethylene-propylene block copolymer and a specific ethylene-propylene rubber and furthermore, if necessary, an unsaturated carboxylic acid-modified polybutadiene or a styrene-based elastomer in a specific proportion.
Polypropylene-based resins are widely used in various fields since they have excellent mechanical properties, and are inexpensive and easily moldable. With these polypropylene-based resins, however, difficulties are encountered in applying fabricating techniques such as adhesion, printing and coating on the surface of a mold since they are chemically inert, have no polar group in the molecule, and have poor solubilities in solvents.
In general, therefore, the surface of a mold has been subjected to specific pre-treatments such as a sand blast treatment, a flame treatment, a corona treatment and an oxidation treatment using a mixed solution of chromic acid and sulfuric acid prior to the coating treatment, for example. These pretreatments, however, are not suitable for practical use since they require very complicated procedures and furthermore the effect is not sufficient.
In order to remove the above described disadvantages of polypropylene-based resins, various methods have recently been proposed. Some of such proposed methods are described below:
(1) An ethylene-propylene rubber is added to high crystalline polypropylene to improve the coating properties thereof as disclosed in Japanese Patent Publication No. 23535/1973. This method, however, suffers from the serious disadvantage that the effect of improving the coating properties is not sufficient unless an oxidation treatment using a mixed solution of chromic acid and sulfuric acid is further applied at low temperatures. Furthermore, the addition of the ethylene-propylene rubber to polypropylene gives rise to the problem that the flowability during molding is reduced.
(2) Polybutadiene is added to polypropylene as disclosed in Japanese Patent Publication No. 42568/1977.
(3) A 1,2-butadiene polymer is added to polypropylene as disclosed in Japanese patent application laid-open No. 153854/1979.
(4) Polybutadiene, and a styrene-based resin or an acrylic polymer are added to polypropylene as disclosed in Japanese patent application laid-open Nos. 789/1980 and 141854/1977.
(5) To the polypropylene resin composition as prepared in (4) above is further added polybutadiene containing functional groups such as hydroxyl groups, carboxyl groups, glycidyl groups, amino groups, and amido groups at the end groups thereof, as disclosed in Japanese patent application laid-open No. 141854/1977.
(6) Polybutadiene and a 1,2-butadiene polymer containing functional groups as described in (5) above at the end groups thereof are added to polypropylene as disclosed in Japanese patent application laid-open No. 27453/1977.
All the methods described in (2) to (6) above have the disadvantage that a degreasing treatment using a solvent is essential to be carried out prior to the coating.
(7) A styrene-butadiene copolymer is kneaded with polypropylene to improve the coating properties of a mold obtained as disclosed in Japanese patent publication Nos. 42568/1977 and 24350/1975. This method, however, has the disadvantage that delamination readily occurs in the mold obtained.